The documentary "S21" gives you that chance, and what do you know  evil's face looks just like everybody else's.

Cambodian director Rithy Panh reunited two of the very few surviving prisoners and more than a dozen jailers in Phnom Penh's S21 prison  now a museum memorializing those who were tortured and murdered there. The men look largely like anybody else you would see on Cambodian streets, and they generally have excuses for their atrocities, which they can still describe in dispassionate detail.

Some are lowly cogs in the killing machine, such as individual cell guards who show how they kept dozens of prisoners bolted to the floor and meted out sips of water and beatings with equal efficiency. At this level, perhaps we can go along with the excuse that these people were just doing a job, serving a party that gave them no choice in the matter. Yet, they did an essential part in the evil that was perpetrated within these walls.

Although one prisoner recalls how the ringing of a phone in the facility gave him encouragement, because it meant he was in the hands of the official state apparatus rather than the unaccountable countryside, consignment to S21 meant guaranteed death. Torturers in the prison kept detailed files on their victims (many are said to have died of "exhaustion" though some are acknowledged to have been killed during interrogation), with the knowledge that all would be pumped thoroughly for confessions and denunciations of others and then disposed of.

Artist Vann Nath with one of his paintings recording his history in S21 prison.

Two former prisoners  who survived only because they were still alive when Phnom Penh was liberated by the Vietnamese 1979  are called on to be the moral anchors of the film. One of them, Chun Mey, breaks down in tears at the very sight of the building and cannot bring himself to go in. The other  a man named Vann Nath with sleek white hair, smooth almond-shaped face and sharp, wise-looking eyes  gives him reassurance before heading inside alone.

Patient beyond belief, Vann Nath draws out the jailers' stories in a way that few others probably would have the inner strength to do. He remains calm and attentive while they coolly recount horrors from the routine mistreatment of inmates bolted to the floor, to murders inhuman enough to make the Nazis' Dr. Mengele smile. His angelic presence is what makes the whole film possible.

"What about 2-year-old babies? Were they enemies of the party?"

 Vann Nath

Only once does Nath seem about to lose his temper. A painter, he has documented the jail's history on canvas, and he confronts the jailers with one of the pictures created from his memory. The guards explain unapologetically that the prisoners were simply enemies of the party  a rationalization that they still cling to after two decades. Nath's eyes flare and his breath comes hard and fast, his anger threatening to burst from his very skin. "What about 2-year-old babies? Were they enemies of the party?" Nath asks, and is met with the same reflexive platitudes again. The guards have found it convenient not to rethink what they've done. They still remember their motivational slogan ("We will be determined and successfully guard the enemy for the party! Determined! Determined! Determined!") and chant it together with gusto.

The stories these men tell of their own deeds grow more and more monstrous, but what's especially interesting is their own dispassion about it. They have the untroubled straightforwardness of men who were just doing their jobs, as if they had been mechanics fixing jeeps. Today they are ordinary farmers or city workers who happen to have rape, torture and murder in their pasts. One of the chief officers points out that at least he wasn't a thief. Another  reminded of the bizarre sense of romantic attachment he felt for an attractive girl he helped torture and kill  notes that they were practically kids, as young as 13, when pressed into service, which helps explain the brew of power, sexual awakening, peer pressure, dislocation, intoxication and indoctrination that underlay their little culture of atrocity, and even humanizes the perpetrators a little. In peaceful times, you might just call this high school; in societies out of control, it's part of what moves ordinary people to participate in genocide.

The prison is now a museum commemorating the thousands killed there by the communist regime.

These people's very ordinariness is part of the movie's message. Culled from the farms and streets, these young men did their country's dirty work and then returned to normal life as casually as if they'd been boy scouts on a camping trip. The work of genocide is not carried out by demons  it's carried out by us, our friends and neighbors, when that particular madness takes hold. The defense against it is courage in the face of conformity, what Vaclav Havel in another context called "living in truth." The cure for it is to expose its face  not a caricature of evil, but the face of ordinary man  to the light. That's what Rithy Panh's jaw-dropping documentary does.

"This is not like if you step over a puddle and get your pants wet, then dry it off and forget about it," Nath says in the film. "Even after 20 years it hasn't been so long. This hasn't dried yet."